1. Field of the Invention
The present invention relates generally to antennas and, more particularly, to paraboloidal grid antennas.
2. Description of the Related Art
An especially useful configuration for an antenna reflector is that of a paraboloid which is generated by rotating the arc of a parabola about its axis. In a feature of this structure, electromagnetic energy transmitted from the paraboloidal focus to the paraboloidal surface is collimated or, equivalently, received collimated energy is reflected from the paraboloidal surface to the paraboloidal focus. One performance characterization of paraboloidal grid antennas is the front-to-back ratio which is a ratio of maximum gain in the antenna's forward hemisphere to maximum gain in its rear hemisphere. This ratio is typically approximated by a power ratio of the main lobe to the rear lobe.
Paraboloidal reflectors have been constructed by replacing a solid paraboloidal surface with one formed by parallel grid members that are aligned with the polarization of a received signal. The grid members are spaced by a common space that is typically calculated to realize a selected front-to-back ratio. Although this replacement generally reduces total aperture efficiency (e.g., from a range of 0.5 to 0.7 to a range of 0.45 to 0.65) and degrades front-to-back ratio (e.g., on the order of 3 dB), it significantly lowers weight and wind loading and reduces the difficulty and cost of antenna installation.
An early description of paraboloidal grid reflectors is found in U.S. Pat. No. 2,850,735 to Harris. In order to reduce the rear lobe and thereby enhance the front-to-back ratio, U.S. Pat. No. 4,801,946 to Matz elongated each grid member in a direction parallel to the paraboloidal axis. In order to further reduce wind loading, U.S. Pat. No. 4,405,928 to Elsbernd provided the grid members with streamlined cross sections.
Modern communication systems (e.g., terrestrial digital video delivery systems) have increased the performance requirements of paraboloidal grid antennas. For example, cellular communication systems with advanced modulation techniques (e.g., quadrature amplitude modulation) and closely-spaced multiple transmitters require high front-to-back ratios (e.g., &gt;26 dB) for subscriber antennas in order to avoid unacceptable co-channel interference and mulitpath reception.
Conventional paraboloidal grid antennas that can meet these front-to-back ratios require a large number of grid elements which increases their manufacturing cost. Because this also increases their weight and wind loading, they require more complex support structures which not only are more expensive but increase antenna installation costs.